Solution-based fabrication of photovoltaic cell

ABSTRACT

An ink for forming CIGS photovoltaic cell active layers is disclosed along with methods for making the ink, methods for making the active layers and a solar cell made with the active layer. The ink contains a mixture of nanoparticles of elements of groups IB, IIIA and (optionally) VIA. The particles are in a desired particle size range of between about 1 nm and about 500 nm in diameter, where a majority of the mass of the particles comprises particles ranging in size from no more than about 40% above or below an average particle size or, if the average particle size is less than about 5 nanometers, from no more than about 2 nanometers above or below the average particle size. The use of such ink avoids the need to expose the material to an H 2 Se gas during the construction of a photovoltaic cell and allows more uniform melting during film annealing, more uniform intermixing of nanoparticles, and allows higher quality absorber films to be formed.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.10/782,017 filed Feb. 19, 2004 now U.S. Pat. No. 7,663,057. Thisapplication is also related to commonly-assigned co-pending applicationSer. No. 10/782,545, titled “HIGH THROUGHPUT SURFACE TREATMENT ON COILEDFLEXIBLE SUBSTRATES” filed Feb. 19, 2004. Both applications are fullyincorporated herein by reference for all purposes.

FIELD OF THE INVENTION

This invention is related to photovoltaic cells and more particularly tofabrication of IB-IIIA-VIA active layers for such cells.

BACKGROUND OF THE INVENTION

Low-cost production of solar cells on flexible substrates using printingor web coating technologies is promising highly cost-efficientalternative to traditional silicon-based solar cells. Recently,solution-based solar cells fabricated from alloys of copper (Cu) andindium (In) with selenium (Se) or sulfur (S) have been developed. SuchCIGS solar cells have been fabricated using a non-vacuum process inwhich a water-based precursor ink is formulated containing mixed oxidesof Cu, In and Ga, which is then coated on rigid or flexible substrates(see U.S. Pat. No. 6,127,202, issued October 2000 to Kapur et. al., andU.S. Pat. No. 6,268,014 issued July 2001 to Eberspacher and Pauls). Theresulting oxide mixture was then subject to reduction in H₂/N₂ mixtureand selenization in an H₂Se/N₂ mixture at high temperatures between 400to 500° C. The resulting CIGS solar cells typically have efficiency inthe range of 8 to 11%. Another alternative ink-based approach usedmetallic powder paste to coat on substrates followed by selenizationunder H₂Se/N₂ at high temperature to form a CIS (or CIGS cell) solarcell (Kapur, V. K., et. al. Sol. Energy Mater Sol. Cells, 60 (2000)127-134 and Kapur et al, Thin Solid Films, 431-432 (2003) 53-57 and alsoKaelin, M., Meyer, T., Kurdesau, F., Rudmann, D., Zogg. H. and A. N.Tiawri. Low Cost Cu(In, Ga)Se₂ Absorber Layers from Selenization ofPrecursor Materials, 3rd World Conference on Photovoltaic EnergyConversion, Osaka, Japan, 2003). Unfortunately, the high temperaturereduction and selenization steps used in each of these solar cellfabrication processes are neither cost effective nor easily scaled tohigh-volume production. Specifically, the H₂Se gas used is both highlytoxic and flammable. Thus, when H₂Se is used at high temperatures,safety, environmental impact, and overall cost are serious concerns inthe manufacturing process. Furthermore, the high temperature reductionand selenization steps make it impractical to make CIGS solar cells oninexpensive polymer or metallized polymer substrates that would warp,melt, or otherwise degrade at such high temperatures in the presence ofreducing and/or oxidizing agents.

Although it is possible to make CIGS-based inks without using oxides,such inks have been produced in the prior art have several drawbacks forhigh-volume, roll-to-roll processing. For instance, the use of bulkCuInSe₂ as a starting material is challenging as bulk CuInSe₂ has amelting point around 1000° C. However, since most of the flexiblesubstrates such as Al foils and plastic foils cannot withstand such ahigh temperature, it is not possible to melt bulk material directly ontoa substrate. Even glass will have serious warping problems at thistemperature and substrate warping typically leads to inefficient cellperformance—so even with deposition onto glass, it is very difficult tocreate high-performance solar cells by melting bulk material. Moreover,the energy requirements needed for high temperature manufacturing at1000° C. will incur substantial cost. Consequently, processes occurringat much lower temperatures are preferred. However, annealing at a lowertemperature tends to hinder the manner of crystal grain growth that iscritical for the proper electronic properties of CIGS solar cell.Certain fluxing agents have been used to reduce the melting point andsintering temperature for CuInSe₂ (A. Vervaet et al. in 9^(th) EuropeanCommunities PV Solar Energy Conference, 1989, 480). Unfortunately, suchfluxing agents can introduce unwanted crystalline phases and alter theelectronic properties of CIGS, thus lowering the efficiency of a CIGSsolar cell.

Thus, there is a need in the art, for a non-oxide based precursor inkthat overcomes the above disadvantages.

BRIEF DESCRIPTION OF THE DRAWINGS

The teachings of the present invention can be readily understood byconsidering the following detailed description in conjunction with theaccompanying drawings, in which:

FIG. 1 is a schematic diagram of a spay co-precipitation apparatus forfabrication of nanoparticles according to an embodiment of the presentinvention.

FIG. 2 is a schematic diagram of a photovoltaic cell according to anembodiment of the present invention.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

Although the following detailed description contains many specificdetails for the purposes of illustration, anyone of ordinary skill inthe art will appreciate that many variations and alterations to thefollowing details are within the scope of the invention. Accordingly,the exemplary embodiments of the invention described below are set forthwithout any loss of generality to, and without imposing limitationsupon, the claimed invention.

As used herein, the following terms have the following meanings:

“Elemental” refers to a material in substantially pure form as opposedto combined with other elements as in an alloy or in a chemicalcompound.

“Non-oxide” refers to a chemical moiety substantially devoid of oxygen.

Embodiments of the present invention provide several key features of aliquid ink that impact cell structure and function. The liquid inkincludes particles containing elements of groups IB, IIIA and(optionally) VIA, e.g., copper and indium (with or without gallium) andselenium or sulfur. The particles may be between about 0.1 and about 500nm. The decreased particle size can significantly lower both the meltingpoint and the sintering temperature required (see e.g., C R M Wronski,“The Size Dependence of the Melting point of Small Particles of Tin” inthe British Journal of Applied Physics vol. 18 No. 12, (December 1967)pp 1731-1737, IOP Publishing, Bristol, UK; L. H. Allen, “NanocalorimetryStudies of Materials: Melting Point Depression and Magic Nanostructures”NNUN Abstracts 2002/Materials, Physics, Processes & Characterization, pp40; Zhang et al., 2000. “Size-dependent melting point depression ofnanostructures: Nanocalorimetric measurements.” Phys. Rev. B 62 (15):548-557; Lisecki et al. 2000. “Annealing Process of Anisotropic CopperNanocrystals.” 2. Rods. Langmuir 16: 8807-8808). Generally, reduction inthe melting point is inversely proportional to the particle radius,i.e., the smaller the nanoparticles, the lower the melting point.Smaller particles also tend to pack closer together and make bettercontact with each other. Reduction in size from bulk material toparticles in about the 500 nm regime can already show significantdifferences in melting point and other altered physical and chemicalproperties. With much smaller particle sizes, e.g. in the nanometer sizerange, the surface area of particles will increase and nanoparticleswill be in intimate contact with each other. In addition, in thenanometer size range, the reactivity of the particles and interactionbetween nanoparticles will be enhanced. This will help particles fusetogether much easier thus enhancing the cohesion of the resulting CIGSlayer (W. H. Qi, et al. in “China—EU Forum on Nanosized Technology”Beijing, P.R.China. December 2002. pp 86-92). This promotes coalescencebetween neighboring particles during sintering.

The particle sizes in the ink are typically distributed over arelatively narrow range, e.g. with the majority of particles of a giventype being within about 40% of an average particle size for that type.Note that for an ink containing particles of different chemical types,e.g., different elemental metals, or different binary combinations, thedifferent types of particles may have different average sizes so thatthe particles in the ink all melt at about the same temperature. Byappropriately adjusting the particle size distribution amongst theparticles of different materials in the mixture, it is possible for allthe particles to melt at about the same temperature during sintering andyield more uniform crystalline phases. This enhances the electronicproperties of the crystals in the resulting CIGS film. By contrast, inthe prior art, CuInSe₂ powders generated, e.g., by a milling ornebulizing process to create powder precursors have typically containeda mixture of both small and large particles, resulting in a relativelybroad size distribution. The presence of such a broad size distributionresults in poor film formation. In particular, smaller particles tend tomelt first while big particles remain unmelted. Further, particles ofdifferent sizes can non-uniformly distribute within a film. Thisheterogeneity leads to defects in film growth, decreases the uniformityand size of crystal grains, and negatively impacts the electronicproperties (e.g., resistivity, bandgap, and carrier transport) of theCIGS layer upon incorporation into a photovoltaic device such as a solarcell. Although the prior art has addressed the advantages of a smalleraverage particle size in IB-IIIA-VI nanoparticle inks (see e.g., U.S.Pat. No. 6,126,740 to Schulz), the resulting active layers made byannealing such inks have often been of relatively poor quality. Theinventors have recognized that the problem is not just the particle sizebut also the particle size distribution, an issue that Schultz does notaddress.

Therefore, according to embodiments of the present invention, an ink forforming an active layer for a photovoltaic cell can be fabricated byforming liquid containing nanoparticles (e.g., elemental metalnanoparticles, non-oxide quantum nanoparticles, or metallicnanoparticles) of about 0.1-nm to about 500-nm diameter containingelements from groups IB, IIIA and VIA. A film of the ink can be spreadonto a substrate, and annealed to form a layer of IB-IIIA-VIA material.The nanoparticles may have a narrow size distribution characterized byan average nanoparticle size D. For example, the nanoparticles used tomake up the film may have sizes within about 40% of D. If the averageparticle size is less than about 5 nm, then the variation can be bigger,e.g., between about 1 nm and about 2 nm. In this case, the resultingrange of melting points is still sufficiently small to be considered tohave a narrow particle size distribution. By way of example, theparticles in the liquid may include Cu with In or Ga and Se or S in astoichiometric ratio of approximately CuIn_(1-x)Ga_(x)(S, Se)₂, where xis between 0 and 1.

Quantum nanoparticles are an important class of nanoparticles that canbe used to make inks according to embodiments of the present invention.The electronic and optical properties of metals and semiconductors canchange dramatically as the particulates of the materials are reduced toapproach the nanometer size range of dimensions. At such size levels,the physical dimensions of the material generally impact the electronic,optical, chemical, and/or physical behavior of the material. Therestriction of an electronic wave function to smaller and smallerregions of space within a particle of material (the resonance cavity) isreferred to as “quantum confinement”. Metal or semiconductor structuresin the nanometer size range, exhibiting the characteristics of quantumconfinement, are commonly referred to as “quantum dots” when theconfinement is in three dimensions, “quantum wires” or “quantum rods”when the confinement is in two dimensions, and “quantum wells” when theconfinement is in one dimension.

Quantum nanoparticles generally form in a size regime where the averageparticle diameter is less than about 10 nm. This small size results innew quantum phenomena that can change dramatically relative to acorresponding bulk material in part because quantum effects arise fromthe confinement of electrons and “holes” in the material (a holerepresents the absence of an electron; the hole behaves as though itwere a positively charged particle). The small particle size alsochanges other material properties, such as the electrical and nonlinearoptical properties of a material, making them very different from thoseof the material's bulk form. For example, the optical bandgap of aquantum nanoparticle can often differ significantly from that of thematerial's bulk form. Further, the increased surface area to volumeratio of quantum nanoparticles renders them more reactive than thematerial's bulk form, often resulting for example in a dramatic meltingpoint depression.

It should also be understood that group IB, IIIA, and VIA elements otherthan Cu, In, Ga, Se, and S may be included in the description of theIB-IIIA-VIA alloys described herein, and that the use of a hyphen (“-”e.g., in Cu-Se or Cu-In-Se) does not indicate a compound, but ratherindicates a coexisting mixture of the elements joined by the hyphen.Where several elements can be combined with or substituted for eachother, such as In and Ga, or Se, and S, in embodiments of the presentinvention, it is not uncommon in this art to include in a set ofparentheses those elements that can be combined or interchanged, such as(In, Ga) or (Se, S). The descriptions in this specification sometimesuse this convenience. Finally, also for convenience, the elements arediscussed with their commonly accepted chemical symbols. Group IBelements suitable for use in the method of this invention include copper(Cu), silver (Ag), and gold (Au). Preferably the group IB element iscopper (Cu). Group IIIA elements suitable for use in the method of thisinvention include gallium (Ga), indium (In), aluminum (Al), and thallium(Tl). Preferably the group IIIA element is gallium (Ga) or indium (In).Group VIA elements of interest include selenium (Se), sulfur (S), andtellurium (Te), and preferably the group VIA element is either Se or S.

A nanoparticle-based CIGS ink has great utility for low cost web coatingof ink-based photovoltaic cells. Methods to make nanoparticles of thedesired materials having the desired narrow particle size distributioninclude controlling the reaction conditions under which thenanoparticles are made or using size-selective precipitation and othertechniques (such as ultrafiltration and membrane emulsification).Nanoparticles in different categories may be incorporated into CIGSinks. These categories include but are not limited to: (1) Ternarynanoparticles such as CuInSe₂ or CuInG nanoparticles; (2) Binarynanoparticles such as CuSe and In₂Se₃ nanoparticles; (3) Metallicnanoparticles such as Cu and In nanoparticles; (4) A suspension madeusing one or more liquid metals, such as Ga; (5) A liquid made using oneor more organometallic precursors. Significantly, each of theseapproaches may be implemented without selenization of the particles,film or IB-IIIA-VIA layer using H₂Se; (6) Metal halides dissolved inchelating agents; and (7) Metal salts. All of these techniques may beused to form a liquid ink. In addition, the use of non-oxide orelemental metal particles avoids having to reduce the particles, film orIB-IIIA-VIA layer with H₂.

The following examples illustrate several different ways, among othersfor creating nanoparticles with a narrow size distribution in thedesired size range.

1. Ternary IB-IIIA-VIA Non-Oxide Quantum Nanoparticles (e.g., Cu(In,Ga)Se(S)₂)

According to one set of embodiments of the present invention, ternary,non-oxide quantum nanoparticles (e.g., quantum dots or quantum rods) formaking a IB-IIIA-VIA ink may be fabricated by several differentapproaches.

1.1 Preparation of Organic Capped CuInSe₂ Non-Oxide QuantumNanoparticles

Non-oxide quantum nanoparticles of CuInSe₂ may be fabricated accordingto a chemical reaction of the type:(IB)(Hal)+(IIIA)(Hal)+TOP(VIA)+TOPO→IB-IIIA-VIA,where IB refers to an element of group IB, IIIA refers to an element ofgroup IIIA, VIA refers to an element of group VIA and Hal refers to ahalogen, preferably iodine (I) or chlorine (Cl). By way of example, IBcan be Cu, IIIA can be Ga or In and VIA can be Se or S. In a particularexample, among others, the reaction may be of the form:CuCl+InCl₃+TOPSe+TOPO→CuInSe₂

The size distribution of the particles may be adjusted by controllingthe reaction conditions under which the nanoparticles are made. By wayof example, Trioctylphosphine oxide (TOPO) (20 g) may be heated to about100° C., followed by degassing and flushing with nitrogen. An equimolarsolution of InCl₃ (2.20 g, 0.01 mol) and CuCl (1.0 g, 0.01 mol) inTrioctylphosphine (TOP) (15 mL) is injected into the hot TOPO at 100° C.The colorless TOPO turned bright yellow after injection, and thetemperature drops to 80° C. The reaction may proceed for about 30 min toone hour, after which the temperature is increased to 250° C., and then1.0 M trioctylphospine selenide (TOPSe) (20 mL) is injected into theTOPO solution (a mixture of Se powder (1.66 g) and TOP (20 mL) isstirred at room temperature for 48 h to give a clear solution (1 M) ofTOPSe. There may be an initial drop in temperature to 220° C., afterwhich the reaction stabilizes at about 250° C. and proceeds for 24-36 h.The reaction mixture is then cooled to about 60° C., followed by theaddition of excess methanol to flocculate the particles. There may be noimmediate visible precipitation on the addition of methanol. A fineprecipitate typically appears after stirring overnight. The precipitatemay be separated by centrifugation and washed with methanol to removethe excess TOPO and then dissolved in toluene. (See e.g., M. A. Maliket. al. Adv. Mater. 1999 11(17), 1441). Nanoparticles in the desiredsize distribution may be obtained from the precipitate, e.g., bysize-selective precipitation or ultrafiltration. TOPO-capped CuInSe₂nanoparticles may be mixed with water and an optional dispersant in anaqueous liquid to form a CIGS ink. The ratio of nanoparticles to waterin weight percentage may be between about 10% and about 60%, e.g. about40%. After coating a substrate with a film of the ink, the TOPO may bedecomposed when heated at elevated temperature. At the same time,CuInSe₂ nanoparticles sinter together to form a polycrystalline film.

1.2 Preparation of CuInSe₂ Non-Oxide Quantum Nanoparticles with aVolatile Capping Agent

TOPO is a relatively large organic molecule, and since it can bedifficult to completely remove large molecules like TOPO by simplethermal decomposition, so carbon contamination can be a potential issuein device fabrication. To overcome this disadvantage, a more preferredembodiment uses a smaller molecule with a lower boiling point, such aspyridine, as a coordinating ligand and solvent. In prior art, pyridinehas been demonstrated to retard nanoparticle growth and narrow the sizedistribution in the case of CdSe nanoparticles. In that prior art,pyridine loosely caps the nanoparticles and ripens the nanoparticlescontinuously. This same principle can be applied to CIGS nanoparticles,which may be fabricated according to a reaction of the type:(IB)(Hal)+(IIIA)(Hal)+Na₂(VIA)+Ligand/Capping Agent→IB-IIIA-VIA,where IB, IIIA, VIA and Hal have the meanings described above. As usedherein, a capping agent can be broadly defined as a compound that bindsto metal. General classes of ligands/capping agents include, but are notlimited to phosphines, amines, alcohols, thiols, ethers, water, andglycols. By way of example, the ligand/capping agent can be pyridine,trioctylphosphine oxide (TOPO), trioctylphosphine sulfide,trioctylphosphine selenide, trioctylphosphine (TOP), triphenylphosphine,pyridine, alcohols (methanol, ethanol, propanol, butanol), thiols,ethane thiol, tetrahydrofuran, ethers, ammonia, amines (methyl amine,ethylamine, ethanolamine, ethylenediamine, trioctyl amine),ethylenediaminetetraacetic acid (EDTA), glycols, amines, diketonates,carboxylic acids, and/or acetonitrile. In a preferred embodiment, theligand/capping agent is pyridine.

By way of example, the reaction may be of the type:CuCl (or CuI or CuCl₂)+InCl₃ (or InI₃)+Na₂Se+Pyridine→CuInSe₂.

By way of example, and without limitation, CuCl (or CuI or CuCl₂) andInCl₃ (or InI₃) with a Cu/In molar ratio ranging from 0.8 to 1.0 may bemixed in alcohol (methanol or ethanol). The concentrations for Cu and Inions may range from about 5 to 10 mM. Na₂Se may also be prepared inalcohol with a concentration double that of the Cu ions. The twoalcoholic solutions are then added into a round-bottom reaction flaskwith pyridine. The precipitated nanoparticles are collected byfiltration or centrifugation. The nanoparticles are then washed withorganic solvents and redissolved in pyridine. The resulting CuInSe₂nanoparticles are nanometer in size, with a narrow particle sizedistribution. Since pyridine is volatile, upon heating the coated filmmade from this nanoparticle ink has carbon-free nanocrystals, withminimal carbon contamination. Further sintering and annealing at lowertemperature yields crystalline grains with uniform phases. (see e.g., B.A. Ridley et. al. Science, 1999 286, 746).

1.3. Preparation of Non-Oxide Quantum Nanoparticles from Single-SourcePrecursors

In other embodiments of the present invention, ternary IB-IIIA-VIAnanoparticles may be prepared from single-source precursors. Forinstance, CuInS₂ nanoparticles may be prepared using the precursor(PPh₃)₂CuIn(SEt)₄. By way of example, dioctyl phthalate (10 mL) may beheated to about 125° C. under vacuum for about 1 h to dry and degas thesolvent. The flask may be back-filled with argon and cooled to roomtemperature prior to adding the precursor. (PPh₃)₂CuIn(SEt)₄ (2.0 g, 2.0mmol) is then added to the reaction flask and heating is resumed. Atapproximately 150° C. the precursor dissolved to form a transparentyellow solution. Typically, no further change is observed with time atthis temperature. At about 150° C., the precursor remains, for the mostpart, intact.

Upon increase of the temperature to about 200° C. a red powder begins toprecipitate within a few minutes. A reaction time of about 2-3 hours maybe employed to complete the precipitation and maximize the yield. Aftercooling to room temperature under argon, toluene (40 ml) may be added tothe reaction mixture to lower the viscosity of the reaction mixture andthe red powder may be isolated by centrifugation. The powder may beconsecutively washed with toluene and then methanol to remove anyreaction byproducts and unreacted starting material, and then driedunder vacuum at about 60-80° C. The washing and centrifugation steps maybe carried out in ambient atmosphere. Then the product may be collectedand stored in a nitrogen-filled glovebox. A portion of the product (100mg) may be placed in a flask containing fresh dioctyl phthalate (10 mL),heated to higher temperatures, e.g. 250 to 300° C., and held at thattemperature for about 1 hour. At a temperature of about 250° C., abrown/black powder (CuInS₂) is obtained. At about 300° C. the resultingpowder is black and contains a CuInS₂ nanoparticulate. (See, e.g., S. L.Castro et. al. Chem. Mater. 2003, 15, 3142)

Approximately the same procedure as outlined above may be followed for aCuInSe₂ precursor. The precursor may be dissolved in the dioctylphthalate at 138° C. For example, (PPh₃)₂CuIn(SePh)₄ (1.951 g, 1.47mmol) may be dissolved in about 10 mL of dioctyl phthalate. A red powder(0.523 g) may be collected after about 2 hours of heating at 200 to 300°C. This power contained a CuInS (Se)₂ nanoparticulate.

The size distributions of the CuInS₂ or CuIn(Se)₂ nanoparticulates maybe adjusted by size-selective precipitation, ultrafiltration and thelike.

1.4. Preparation of CuInS(Se)₂ Nanoparticles using SprayCo-Precipitation

An alternative embodiment to reduce particle size and narrow theparticle size distribution utilizes a spray co-precipitation technique.In the prior art, ZnGa₂O₄ nanocrystals with a mean particle size lessthan 10 nm have been made using spray co-precipitation. In a sprayingco-precipitation apparatus 100, as shown in FIG. 1, compressed inertgas, e.g., nitrogen from gas sources 102, 103 drives reactants fromreactant sources 104, 106 very quickly through stainless steel pipelines108, 110. The flow rate can be adjusted by a flowmeter from about 5 toabout 20 m/s. In general, one of the reactants may include one or moremetal salts. As used herein the term metal salts includes metal halides,metal acetates, metal sulfates, metal nitrates, metal alcholates, metalcarbonates, metal phenolates and/or organometallics. In a particularexample, the reactants may include, a metal halide, such as CuHal/InHal(where Hal refers to I or Cl) and thiourea or selenourea. The reactantsmix and react at a T joint 112 of the two pipelines 108, 110 and thenspray into a beaker 114 with solvents (e.g., water or alcohols). Themolar ratio of Cu, In and thiourea or selenourea may be adjusted e.g.,by controlling the flow of the reactants with valves 116, 118, 120, 122to form CuInS₂ nanocrystals 124. The size distributions of thenanocrystals 124 may be adjusted by size-selective precipitation,ultrafiltration and the like. For further information see Z. Jiao et.al. Sensors 2003, 15, 3142.

2. Production of Binary Nanoparticles

Some of the embodiments described above can be modified to producebinary non-oxide quantum nanoparticles, e.g., IB-VIA or IIIA-VIA orIB-VIA nanoparticles. These may be mixed together in the liquid toprovide the desired ratios of IB, IIIA and VIA elements. By way ofexample there are several different approaches to producing CuSe andIn₂Se₃.

2.1. TOPO-Capped CuSe Non-Oxide Quantum Nanoparticles

In another embodiment, quantum dots with nearly uniform size may befabricated by solvothermal methods according to a reaction of the type:(IB)(Et₂CN(VIA)₂)₂+TOPO→IB-VIA,where IB, IIIA, VIA and TOPO have the meanings describe above. By way ofexample IB may be Cu, and VIA may be Se or S.

In a particular example, the quantum dots may be fabricated using areaction of the type:Cu(Et₂CNSe₂)₂+TOPO→CuSe.

Quantum dots and rods of this type have been synthesized in the range ofabout 5 to about 15 nm using Solvothermal or solution reaction methods(see, e.g., M. A. Malik et. al. “A Novel Route for the Preparation ofCuSe and CuInSe₂ Nanoparticles,” in Advanced Materials, Vol 11, No. 17,pp 1441-1444, 1999). By way of example, and without limitation, CuSeQuantum Dots may be fabricated as follows. Cu(Et₂CNSe₂)₂ (0.5 to 2 g)may be dissolved in tri-n-octylphosphine TOP (10 to 30 mL). Thissolution is then injected into hot tri-n-octylphosphine oxide TOPO (10to 20 g) at 250° C. A decrease in temperature of 20-30° C. may beobserved. The solution is then allowed to stabilize at about 250° C. andis heated for about 60 min at this temperature. The solution is thencooled to approximately 70° C., and after an excess of methanol wasadded, a flocculant precipitate forms. The solid may be separated bycentrifugation, washed with methanol, and redispersed in toluene. Thetoluene may then be removed under a vacuum to give pale yellowTOPO-capped CuSe nanoparticles. The particles may be washed three timeswith methanol and redissolved in toluene. Size-selective precipitationmay then be carried out on the particles to obtain a collection ofquantum dots of the desired average particle size and size distribution.

2.2. CuSe Non-Oxide Quantum Nanoparticles Prepared in Volatile Solvent

Pyridine-capped binary nanoparticles (e.g., CuSe) may be preparedaccording to a reaction of the type:(IB)(Hal)+Na₂(VIA)+ligand/capping agent→IB-VIA+2Na(Hal)

By way of example, IB may be Cu, Hal may be Cl or I, VIA may be Se or S.The ligand/capping agent may be any ligand that is co-processable withthe metal salts used. By way of example, the ligand/capping agent may bepyridine, trioctylphosphine oxide, trioctylphosphine,triphenylphosphine, pyridine, alcohols (methanol, ethanol, propanol,butanol), ethane thiol, tetrahydrofuran, ethers, ammonia, amines (methylamine, ethylamine, ethylenediamine) and/or acetonitrile. In a particularexample, the reaction may be of the type:CuCl₂+Na₂Se+Pyridine→CuSe+2NaIThe conditions for this reaction are similar to those described insection 1.2 except that there is no In ion involved in the reaction.

2.3. In₂Se₃ Non-Oxide Quantum Nanoparticles Prepared in Volatile Solvent

In₂Se₃ nanoparticles may be prepared according to a reaction of the type(IIIA)(Hal)+Na₂(VIA)→IIIA-VIA+Na(Hal).Where IIIA, VIA and Hal can be any of the appropriate elements describedabove. By way of example, the reaction may be of the type2InI₃+3Na₂Se→In₂Se₃+6NaI

The conditions for this reaction are similar to those described insection 1.2 except that there is no Cu ion involved in the reaction.

2.4. Sonochemical Synthesis of CuSe, In₂, Se₃ or CuInSe₂ Non-OxideQuantum Nanoparticles

In the prior art, ZnSe nanoparticles with a mean size of 3 nanometershave been prepared using a sonochemical method (see, e.g., J. Zhu et.al. Chem. Mater. 2000 12 73). According to an alternative embodiment ofthe invention, the same sonochemical method can be applied to formingCuSe or In₂Se₃ or CuInSe₂ nanoparticles for a CIGS-based ink. By way ofexample, and without limitation, Cu(Ac)₂ (445 mg) and/or In(Ac)₃ (720mg) and 240 mg of selenourea may be dissolved in 70 mL of doublydistilled water and sonicated for about 1 hour under an atmosphere ofargon, at room temperature. A round-bottom Pyrex glass vessel (totalvolume 70 mL) may be used for the ultrasound irradiation. Thetemperature of the mixture rises gradually to about 80° C. duringultrasonic irradiation. After about 1 hour of sonication, the solutionmay be centrifuged, and the precipitate washed with water and then withabsolute ethanol.

3. Elemental Metal Nanoparticles of Cu, In and Cu-In alloy with Se(S)Vapor or Powders

In a preferred embodiment, the nanoparticles for the ink may be preparedfrom a mixture of metal nanoparticles and a vapor or powder containingeither Se or S. Metal nanoparticles have been prepared by a wide varietyof techniques including but not limited to laser ablation, nucleationfrom vapor, exploding wires by electrical current surge, thermaldecomposition of organometallic compounds, sonolysis, pulse radiolysis,electrochemical reduction and chemical reduction of the correspondingmetal salts, and the like. Reduction of metal salts in the presence of asuitable protecting agent is a commonly used technique. Generally, areductant, such as borohydride, hydrotriorganoborates, hydrogen orcitrate, is added to a solution of the corresponding metal salt. Aneasily oxidized solvent may function both as the electron donor and thedispersing medium. Such particles are of particular interest forcatalysis, as the metal surface is readily accessible. Alcohols andethers have been quite extensively used for this purpose. Metalnanoparticles have a tendency to agglomerate, and therefore, it isnecessary to protect them using surfactants or polymers, such ascyclodextrin, PVP, PVA, citrate or quaternary ammonium salts. In anotherstrategy, the metal salt is reduced in micelles. A wide variety ofultra-fine colloidal surfactant-micelle protected metal nanoparticlescan be successfully synthesized in solution. The size distribution ofthe metal nanoparticles may be controlled, e.g., by size-selectiveprecipitation, ultrafiltration and the like.

Such metal colloids can be prepared in an ink formulation. After beingmixed with Se (or S) fine powders, the ink can be coated on substrateand then subject to heat treatment to drive a reaction of the type:(IB nanoparticle)+(IIIA nanoparticle)+VIA+HEAT→IB-IIIA-VIA

By way of example, IB may be Cu, IIIA may be In and/or Ga, and VIA maybe Se or S. The reaction between metal or metal alloy nanoparticles andSe (or S) powder forms CIGS (or CIS) film. The ink may include ahomogeneous mixture of metallic particles and powders of group VIAmaterials. The individual metallic particles are generally uniformlydistributed throughout a matrix of, e.g., Se or S powder.Sub-micron-sized metallic particles may be formed by methods involvingmechanical comminution, e.g., milling, grinding, crushing and thedesired size distribution obtained, e.g., by ultrafiltration orsize-selective precipitation.

In addition, nanometer-scale particles may be prepared by a gascondensation method. For example, in the gas condensation method, ametal may be heated to a vapor. The vapor then collects and condensesinto particles. Such a technique may produce particles that arenominally spherical, approximately 30-40 nm in diameter and having avery tight size distribution (e.g., ±5 nm to ±10 nm). See also“Ultrafine Metal Particles”, C. G. Granqvist and R. A. Buhrman, J.,Appl. Phys., 47, 2200, (1976).

In another embodiment, the metal particles may be prepared by explodingfine metal wires with a surge of electrical current. For example, metalwires 7.5-25 microns in diameter may be exploded in a liquid such aswater or alcohol by applying an increasing DC voltage across the wireuntil it explodes, e.g., at a few tens of volts. The resulting particlesremain suspended in the liquid. Copper wires exploded in this fashioncan produce particles about 30 to 70 nm in diameter.

The metal nanoparticles and group VIA powder may be mixed together withan optional binder to form a green microstructure. The binder serves tohold the metallic particles together, provides lubrication while theparticles are mixed, and then or vaporizes completely during thesintering process leaving no trace of the binder in the final metallicproduct. Preferably, the binder combusts or vaporizes completely duringthe sintering process leaving no trace of the binder in the final CI(G)Sactive layer. Binders useful in conjunction with such embodiments of theinvention include, but are not limited to, wax- or oil-based binderssuch as Acrawax® C® (Glyco), Advawax® 240, 275, 280, 290 (Carstab),Eramid® (Hexcel), Glucolube® VL (Glyco), Hostalub® FA 1 (Amer. Hoechst),HSTA 1, 3 (Hexcel), Interstab® G-8257, Kantstik® S powder (SpecialtyProd.), Kemamide® W-20, W-40, E (Humko Chem.), Nopcowax® 22DS (SyntheticProd.), Paricin.RTM.285, 220 (NL Industries), Ross Wax® 140 (Frank B.Ross), and Slip-Eze®, Slip-Quick®, Vyn-Eze® (Hexcel) and the like; loweralkanols (C₁-C₆ alkanols, preferably C₁-C₄ alkanols) such as ethanol,isopropanol, and the like; or traditional dispersant/binder systems usedin ceramic possessing, such as polymethacrylate/polyvinylalcohol/polyethylene glycol systems.

In some embodiments, the metal particles may be formed into a pastemixture comprising the elemental metal particles along with a resin,solvent, wetting agent, and flow control agent. In addition, pastes canbe made by combining elemental metals or other metallic particles with apolymer. Furthermore, a nanopowder of metal particles may be added topolymer matrix materials. Examples of suitable polymer matrix materialsinclude poly(methyl methacrylate) (PMMA), poly(vinyl alcohol) (PVA),polycarbonates, polyalkenes, and polyaryls. In addition dispersants,binders, detergents, modifiers, or additives may be included along withthe matrix material. Examples of modifiers or additives includeplastisols, thickeners, or thinners to change the viscosity or change orimprove the smoothness of the ink film surface. For example, “ViscosityBuster 10025VB” from Wilflex, at about a 1% concentration typicallystabilizes and can improve the flow properties of ink. Additionalmodifiers or additives include, e.g., acids or bases to change pH of theink or additives that lower the flash temperature of the ink andretarders or reducers to change the drying time of the ink. Potentialsources of such modifiers and additives include Wilflex, Rutland Inks ofPineville, N.C., Chemical Consultants, Inc. of Corona, Calif., UnionInks of Singapore.

In the event that the nanoparticles are subject to oxide formation onthe surface of the particles, for example, copper particles having acoating of copper oxide, the molding composition may additionallycomprise a fluxing agent. The fluxing agent serves to substantially haltformation of the oxide layer, thereby enhancing the ability of themolding composition to form the green microstructure and aiding in thefinal sintering process. Examples of common fluxing agents in generaluse include abietic acid, stearic acid, hydrochloric acid, aminehydrochlorides and amine resins. Either a suitable commercial fluxformulation or a specially formulated fluxing agent may be utilized.Further, to minimize or eliminate oxide formation on the surface of thenanoparticles, the nanoparticles can be formed in an atmosphere thatlacks oxygen, e.g. a nitrogen-filled glovebox.

4. Liquid Metal Based Inks

In another embodiment, the liquid ink may be made using one or moreliquid metals. For example, an ink may be made starting with a moltenmixture of Gallium and/or Indium (commercially available, e.g., fromAlfa Aesar of Ward Hill, Mass.). Copper nanoparticles may then be addedto the mixture, which may then be used as the ink/paste. Coppernanoparticles are available commercially, e.g., from MaterialsModification Inc., of Fairfax, Va. Alternatively, the temperature of theCu-Ga-In mixture may be adjusted (e.g. cooled) until a solid forms. Thesolid may be ground at that temperature until small nanoparticles (e.g.,less than 5 nm) are present. Selenium may be added to the ink and/or afilm formed from the ink by exposure to selenium vapor, e.g., before,during, or after annealing.

5. Organometallic Liquid Inks

In yet another embodiment, the liquid ink may be made using one or moreorganometallic precursors of IB, IIIA and VIA elements. For instance,examples of copper precursors include Copper II hexafluoropentanedionate(Gelest SKU#AKC253); Copper II methoxide (Gelest SKU#AKC254); Copper IIethoxide (Gelest SKU#AKC250); and Copper II ethylacetoacetate (GelestSKU#AKC252), all of which are available from Gelest, Inc. ofMorrisville, Pa. Examples of indium precursors include Indium2,4-Pentanedionate (Gelest SKU#AKI370), also called indiumacetylacetonate; Indium Methoxyethoxide (Gelest SKU#AKI365); and Indiummethyl(trimethyl)acetylacetate (Gelest SKU#AKI373), also all availablefrom Gelest. Examples of Gallium precursors: Gallium III2,4-Pentanedionate (Gelest SKU#AKG310); and Gallium III Ethoxide (GelestSKU#AKG320), also available from Gelest.

The precursors may be mixed as liquids to form the ink. The organiccomponents may be burned out of the ink by heating at about 500° C.,e.g., after a film of the ink is disposed on a substrate. Alternatively,a sol-gel may be formed from organometallics of e.g., Cu, In, and/or Ga.In general, the sol-gel processing involves the transition of a systemfrom a liquid “sol” (mostly colloidal) into a solid “gel” phase. Thestarting materials used in the preparation of the “sol” are usuallyinorganic metal salts or metal organic compounds such as metal alkoxidesalong with water, alcohol and, optionally, a small amount of acid.

For example, the ink may be fabricated by making a pseudo sol out ofmixed metal organic precursors, such as alkoxides or diketonates ofCopper, indium, gallium in the desired stoichiometric ratio. By way ofexample, copper (II) ethoxide or Copper(II) ethylacetoacetate may bemixed with gallium (III) ethoxide or gallium (III) pentanedionate andwith indium (III) methoxyethoxide or indium (III) 2,4 pentanedionate. Asol may be formed by adding water, an alcohol and, optionally, an acid,to the mixed metal organic precursors. The sol may be coated in a dish(e.g. drop cast), annealed to remove labile groups, and allow drying toform a powder. Note that this process can be scaled to bulk in highvolume. The metallic powder can then be milled and mixed with dispersantand then coated, and the resultant film can then be reduced by treatmentwith H₂ and/or N₂ at 400° C., after which 10-50 mbar Se vapor can beintroduced to selenize the film. Alternatively, the powder compounds canbe first reduced to metals through exposure to H₂ gas, then mechanicallymilled and/or ground up mixed with dispersant and then coated.

6. Metal Halides Dissolved in Chelating Agents

In yet another embodiment, a metal halide (MCl or MI, where M is ametal) could be dissolved in any of a number of chelating agents, then(optionally) mixed in water to keep the metals in solution. The solutionmixture can then serve as a basis for a precursor paste or ink. Suitablemetal halides include chlorides and iodides of the group IB, IIIA andVIA metals. Suitable chelating agents include, but are not limited to,e.g. amines, diamines such as ethylene diamine, polyamines, polyethyleneglycol, low molecular weight alcohols, ketones, diketones, carboxylicacids, and mixtures thereof.

7. Metal Salts

Metal salts and other metal compounds can also be used as reagents tosynthesize metal nanoparticles. Provided that they exhibit properreactivity, suitable metal salts and other metal compounds include butare not limited to metal halogenides, metal nitrates, metal nitrites,metal nitrides, metal sulfates, metal sulfites, metal sulfides, metalalcoholates, metal phosphates, metal phenolates, metal carbonates, metalcarboxylates, and blends or mixtures of these salts. For example, foreach element or non-oxide compound from the group IB, IIIA and VIAmetals, metal salts can be synthesized or purchased (e.g., throughSigma-Aldrich of St. Louis, Mo.), formed or purified asnanoparticulates, then each of the nanoparticulates can be intermixed inthe proper ration to serve as the basis for an ink for effective CIGSabsorber layer function.

Photovoltaic Devices

The nanoparticles are generally mixed with an organic or inorganicsolvent to form the ink. In some embodiments of the invention, thenanoparticles may be mixed with water to form an aqueous suspension thatserves as the basis of the ink. Schulz, by contrast teaches non-aqueouscolloidal suspensions and stresses that every effort should be made toavoid introducing even incidental amounts of water into his colloidalsuspensions in order to avoid agglomeration of particles. In fact,aqueous suspensions have several advantages over non-aqueous suspensionsduring ink formation and deposition. First, water is an inexpensivesolvent, and it is much easier to scale to high-volume productionprocesses. In contrast, organic solvents are inherently more expensiveand hazardous than water, and waste solvents and other organicbyproducts incur costs resulting from waste processing. Second, a widevariety of water-compatible dispersants can easily be added to anaqueous suspension of nanoparticles, which are stabilized by thedispersant, which keeps the nanoparticles separated from each otheruntil the annealing step melts them together. In contrast, if suchwater-compatible dispersants were added to an organic (non-aqueous)solution, such a mixture may form two phases as a result oforganic-inorganic partitioning in the solution. In this case,nanoparticles that were initially stabilized by capping agents maysubsequently collapse and form aggregates, leading to non-uniformcoatings. Examples of suitable dispersants include Tamol™ and Duramax™from Rohm and Haas of Philadelphia, Pa. Further information on Tamol™may be found at http://www.rohmhaas.com/coatings/dispersants.html.Further information about Duramax™ may be found athttp://www.rhcis.com/markets_and_products/electronic.html.

Note that, as used herein, the term “aqueous suspension” refers to theink at the end of its manufacture. The use of the term “aqueous” doesnot exclude the possibility that during manufacture, the particles maybe dispersed in an organic solvent or some solvent other than water. Forexample, if the particles are capped with organic ligands and in some ofthe particle formation chemistries, the particles may be initially madein alcohol or another organic solvent then later precipitated orisolated for resuspension in water to form the ink. Furthermore, in someembodiments of the present invention, the particles may be so small thatthey form liquids, e.g., the particles may be as small as individualatoms or molecules.

The liquid ink (aqueous or otherwise) may be spread in a thin film overa substrate using solution based coating techniques such as web coating,spray coating, spin coating, doctor blade coating, printing techniquesincluding contact printing, gravure printing, microgravure printing,ink-jet printing, jet deposition, and the like. Such a film can bedeposited on a flexible substrate, in a roll-to-roll manner using acommercially available web coating system. The film may then be heatedto remove the solvent and to sinter the nanoparticles together to form alayer of a IB-IIIA-VIA alloy. The alloy may have the general formulaCuIn_(1-x)Ga_(x)(S, Se)₂, where x is between 0 and 1. The reducedtemperature sintering (e.g., from about 200° C. to about 600° C.) madepossible by the small particle size and narrow size distributionfacilitates forming CIGS films in a high volume roll-to-roll productionenvironment on thin flexible substrates such as metal foils ormetallized plastic films. To heat and sinter the film, if deposited on aflexible substrate, the film can be wound into a coil and the coil canbe heated such that the entire roll is heated at the same time,substantially increasing the scaleability of the heating process throughsuch a high-volume batch process. Examples of processing a coiledsubstrate are described e.g., in U.S. patent application titled “HIGHTHROUGHPUT SURFACE TREATMENT ON COILED FLEXIBLE SUBSTRATES”, which hasbeen incorporated by reference.

Some high-volume batch processes for coating the substrate with the ink,e.g., chemical bath deposition (CBD) for liquid-based deposition, andatomic layer deposition (ALD) for gas- and vapor-based deposition, couldpotentially coat both sides of a coiled substrate at one time. However,it may be desirable to primarily coat only one side, since double-sidedcoating can result in waste of valuable components of the ink, e.g.,indium. To avoid such waste, two substrates may be attached together“back-to-back” to form a dual substrate having, in effect, two frontsides. The dual substrate may then be wound into a coil and coated suchthat both front surfaces get coated while the back surfaces do not.Preferably, the substrates are attached in a manner that allows them tobe separated from each other after processing. By way of example thesubstrates may be attached with a low-strength adhesive or electrostaticfilm applied to the back side of one or both substrates. Alternatively,an edge where the two substrates join may be sealed, e.g., with a tape,so that reactants cannot reach the back sides during processing.Processing the substrate in this fashion wastes less of the ink and mayincrease the area of the substrate that can be coated at one time.

The IB-IIIA-VIA alloy layer can be used in the active layer ofphotovoltaic cell, e.g., of the type shown in FIG. 2. The solar cell 200generally includes a substrate or base layer 202, a base electrode 204,a IB-IIIA-VIA layer 206, a window layer 208, and a transparent electrode210. The base layer 202 may be made from a thin flexible materialsuitable for roll-to-roll processing. By way of example, the base layermay be made of a metal foil, such as titanium, a polymer such aspolyimide or a metallized plastic. The base electrode 204 is made of anelectrically conducive material. By way of example, the base electrode204 may be a layer of stainless steel or molybdenum, e.g., about 0.5micron to about 1 micron thick.

By way of example, and without limitation, the IB-IIIA-VIA layer 206 mayinclude material of the general formula CuIn_(1-x)Ga_(x)(S or Se)₂. TheIB-IIIA-VIA layer 206 may be fabricated by depositing a film of ananoparticle-based ink, paste or slurry, e.g., roughly 4 to 5 micronsthick when wet on the base electrode 204. The film may be annealed byheating to a temperature sufficient to burn off any binders or caplayers on the particles and sinter the particles together. TheIB-IIIA-VIA 206 layer may be about 1 micron to about 2 microns thickafter annealing. By using nanoparticles of the average size and sizedistribution described above, the ink, paste or slurry may be annealedat a temperature compatible with the underlying substrate 202 andelectrode 204. An optional adhesion layer 203 may facilitate bonding ofthe electrode 204 to the substrate 202.

After annealing, the film may optionally be exposed to selenium vapor atabout 300-500° C. for about 30-45 minutes to ensure the properstochiometry of Se in the film. To carry out such a Se vapor exposure,the film, if deposited on a flexible substrate, can be wound into a coiland the coil can be coated so that the entire roll is exposed at thesame time, substantially increasing the scaleability of the Se vaporexposure process through such a high-volume batch process, e.g., asdescribed above.

The window layer 208 is typically used to smooth out the “slope” betweenthe bandgaps of the different materials making up the IB-IIIA-VIA layer206. By way of example, the bandgap adjustment layer may include cadmiumsulfide (CdS), zinc sulfide (ZnS), or zinc selenide (ZnSe) or somecombination of two or more of these. Layers of these materials may bedeposited, e.g., by chemical bath deposition, to a thickness of about 50nm to about 100 nm.

The transparent electrode 210 may include a transparent conductive oxidelayer 209, e.g., zinc oxide (ZnO) or aluminum doped zinc oxide (ZnO:Al),which can be deposited using any of a variety of means including but notlimited to sputtering, evaporation, CBD, electroplating, CVD, PVD, ALD,and the like. If the substrate is flexible and the deposition techniqueis ALD or CBD or the like, a coiled/wound flexible substrate can beexposed so that the entire roll is processed at one time, e.g., asdescribed above. The transparent electrode 210 may further include alayer of metal (e.g., Ni, Al or Ag) fingers 211 to reduce the overallsheet resistance.

An optional encapsulant layer (not shown) provides environmentalresistance, e.g., protection against exposure to water or air. Theencapsulant may also absorb UV-light to protect the underlying layers.Examples of suitable encapsulant materials include one or more layers ofpolymers, such as polyethylene terephthalate (PET), ethylene vinylacetate (EVA), and/or Mylar®. Mylar is a registered trademark of E.I. duPont de Nemours and Company of Wilmington, Del. Inorganic materials,such as glass and plastic foils, metalized plastic foils, and metalfoils may also be used for the encapsulant layer. The encapsulant layermay also include nitrides, oxides, oxynitrides or other inorganicmaterials. Alternatively, the encapsulants may include Tefzel® (DuPont),tefdel, thermoplastics, polyimides, polyamides, nanolaminate compositesof plastics and glasses (e.g. barrier films), and combinations of theabove. For example, a thin layer of (expensive) EVA/polyimide laminatedto thick layer of (much less expensive) PET.

Embodiments of the present invention provide for low-cost, high-volumeproduction of large area photovoltaic devices without having to undergoselenization with hazardous H₂Se or reduction of oxides in H₂.Furthermore, embodiments of the present invention are compatible withroll-to-roll manufacturing of photovoltaic cells and modules that can bereadily scaled up to high production volumes.

While the above is a complete description of the preferred embodiment ofthe present invention, it is possible to use various alternatives,modifications and equivalents. Therefore, the scope of the presentinvention should be determined not with reference to the abovedescription but should, instead, be determined with reference to theappended claims, along with their full scope of equivalents. Theappended claims are not to be interpreted as includingmeans-plus-function limitations, unless such a limitation is explicitlyrecited in a given claim using the phrase “means for.”

1. A method for fabricating a solar cell formed using a liquidcontaining intermixed elements of groups IB, IIIA, and VIA, comprisingthe steps of: a) forming by sonochemical method non-oxide nanoparticles;b) providing a suspension comprised of one or more liquid metals,wherein the one or more liquid metals comprises at least one group IIIAbased material; c) mixing the non-oxide nanoparticles of step a) and thesuspension of one or more liquid metals of step b) to form an inkcontaining both the non-oxide nanoparticles and the one or more liquidmetals; wherein the nanoparticles are in a desired particle size rangeof between about 0.1 nm and about 500 nm in diameter, wherein a majorityof the mass of the nanoparticles range in size from no more than about40% above or below an average particle size, or, if the average particlesize is less than about 5 nanometers, from no more than about 2nanometers above or below the average particle size.
 2. The method ofclaim 1 wherein the ink comprises non-oxide nanoparticles of step a)formed from Cu(Ac)₂ and/or In(Ac)₃ and selenourea dissolved in water andsonicated for about 1 hour or more under an atmosphere of inert gas. 3.The method of claim 1 wherein a round-bottom vessel is used for thesonochemical method of step a).
 4. The method of claim 1 wherein thetemperature of the mixture rises gradually to about 80° C. duringultrasonic irradiation in sonochemical method of step a).
 5. The methodof claim 1 wherein after about 1 hour or more of sonication insonochemical method of step a), the solution is centrifuged, and theprecipitate washed with water and then with absolute ethanol.
 6. Themethod of claim 1 further comprising depositing the liquid ink on asubstrate; and heating the liquid ink in one or more steps to form aphotovoltaic absorber layer.
 7. The method of claim 6, wherein thesubstrate comprises a metal foil up to 2 meters wide.
 8. The method ofclaim 6, wherein the substrate comprises of at least one material fromthe group consisting of: stainless steel, molybdenum, and aluminum. 9.The method of claim 6, wherein the substrate comprises a metallizedfilm.
 10. The method of claim 6, wherein applying the liquid inkcomprises spreading a thin film of the liquid ink over the substrateusing solution based coating techniques selected from the groupconsisting of: web coating, spray coating, spin coating, doctor bladecoating, printing techniques including contact printing, gravureprinting, microgravure printing, ink jet printing, jet deposition, andcombinations thereof.
 11. The method of claim 6, wherein applying theliquid ink comprises spreading a thin film of the liquid ink over thesubstrate in a roll-to-roll manner using a web coating system.
 12. Themethod of claim 1 wherein the ink includes a solvent.